Lunar and Planetary Science XXIX 1262.pdf
THE LAKE SAARIJ?RVI - A NEW METEORITE IMPACT STRUCTURE IN NORTHERN FINLAND. Lauri J. Pesonen1, Martti Lehtinen2, Pekka Tuukki3 & Andreas Abels4; 1Laboratory for Paleomagnetism, Geological Survey of Finland, FIN-02150 Espoo, Finland
Summary. Petrographic analysis of drill core samples, In thin sections, some of the quartz clasts show planar de- supported by geophysical and geological data, confirm that formation features (PDFs) with three orientations (011;¯3, Lake Saarij@rvi, Taivalkoski, northern Finland, is a remnant 101;¯3 and 101;¯0/112;¯0; Fig. 1c). Some biotite grains of a deeply eroded impact structure. It is the eighth impact show kink bands and feldspar reveals mosaicism. These structure found in Finland. petrographic results clearly demonstrate that the rock has Background. The Saarij@rvi structure has been a tar- been shocked to a peak pressure over 15 GPa. Petrographic get of investigation for more than 60 years. Three explana- investigations of the samples from the in-situ breccia of the tions for its origin have been offered: (i) a tectonically central island showed also shock effects, but of weaker na- downfaulted sedimentary unit, (ii) a kimberlite pipe, or (iii) ture than observed in the drill core. We conclude that a meteorite impact [1, 2, 3, 4, pers. com. by S. Elo]. In order Saarij@rvi is a deeply eroded remnant of an impact structure, to find out the real nature of the structure, Malmikaivos Oy, somewhat similar to the Iso-Naakkima structure, Finland a Finnish exploration company, carried out a deeper drilling [7]. In the current data base of Finnish impact structures it is into its center (Fig.1a). Here we summarize the morphologi- number eight. The age of the Saarij@rvi impact event is still cal, geophysical, geological, and petrographic data for the unknown, but can be constrained to be older than 0.6 Ga, structure. which is the approximate microfossil age of the subse- Geological setting. The Saarij@rvi impact site (center quently infilled sediment [5], and younger than 2.45 Ga, co-ordinates 65/17,4'N 28/23,3'E) is visible in Landsat sat- which is the probable age of the diabase dykes. If the central ellite images as a roundish structure with a diameter of ~1.5 island, with its breccia outcrops, turns out to be a remnant km. The structure occupies the present lake Saarij@rvi ex- of a central uplift, the original diameter of the structure tending slightly to the north into a topographically low area. must have been much larger than the presently seen diame- Geologically the structure lies in the pre-Svecofennian (Ar- ter of 1.5 km. Indeed, some of the "tails" in the gravity data chaean) basement of the Suomussalmi block. The main hint to a larger structure. We present geophysical and pet- target rock type is tonalitic, gneissose granite rich in oligo- rographic results of the Saarij@rvi structure and discuss its clase. The bedrock is cut by diabase dykes of different ages implications for impact cratering in Fennoscandia. as seen by high-resolution aeromagnetic maps and in out- crops including the central island (Fig. 1a). Within the References. [1] Hyypp@, J. and Y. Pekkala, 1987. Tut- structure proper, on top of the basement, there exists an up kimusty`selostus Taivalkosken kunnassa valtausalueilla Saarij@rvi 1-3 to 156 m thick sedimentary unit with varieties of sandstones suoritetuista savikivitutkimuksista. Geologian tutkimuskeskus, and claystones. The central drilling penetrated the sedi- Raportti M 06/3533/-87/1/89. [2] Elo, S., Jokinen, T. and P. Turunen, mentary pile up into fractured basement gneiss. A few cen- 1983. Geophysical investigations of the Saarij@rvi structure. In: L. timeter thick breccia layer was found at the top of the frac- Eskola (ed.), Geophysics for Geologists, Geol. Surv. Finland, Report of tured basement just below the sediment. Moreover, two Investigation 58: 101-116 (in Finnish). [3] Henkel, H. and L.J. anomalous mafic veins cut the fractured basement, which is Pesonen, 1992. Impact craters and craterform structures in Fennoscan- below them less fractured. Two breccia outcrops were also discovered on the central island (Fig. 1a). dia. Tectonophysics 216: 32-40. [4] Pesonen, L.J., Lehtinen, M., Geophysical observations. Strikingly circular aeroe- Tuukki, P. and A. Abels, 1997. The Lake Saarij@rvi structure, Tai- lectromagnetic and ground electromagnetic anomalies are valkoski - a new meteorite impact crater in Finland. In: P. Kaikkonen, associated with the Lake Saarij@rvi structure (Fig. 1b) [2 ,6]. K. Komminaho and H. Salmirinne (eds.), Negotiating Days of the They are partly related to the lake water and bottom mud of Applied Geophysics, Univ. of Oulu, Finland (22.10-23.10.1997), p. Quaternary age, but partly reflect deeper located conductors, 8-9. [5] Tynni, R. and A. Uutela, 1985. My`h@is-prekambrinen ajoitus presumably breccia layers or fractured bedrock. The aero- Taivalkosken savikivelle mikrofossiilien perusteella. Geologi 4-5: magnetic data show a generally weak signature over the 61-65. [6] Turunen, P., 1995. VLF-R- ja tasa-virtamittaukset Tai- lake, and some of the linear aeromagnetic anomalies due to valkosken Saarij@[email protected] Tyr`raportti, Q 19/3533/1995/1, Geol. Surv. mafic dykes appear to be truncated by the structure. The Finland, Regional Agency, Rovaniemi, 14 pp. [7] Elo, S., Kuivasaari, gravity data reveal a negative Bouguer anomaly of about T., Lehtinen, M., Sarap@@, O. and A. Uutela, A., 1993. Iso-Naakima, a -1.5 mGal over the structure. These geophysical data are consistent with an impact origin of the structure [4]. circular structure filled with Neoproterozoic sediments, Pieks@m@ki, Petrographic observations. Previous petrographic in- southeastern Finland. Bull. Geol. Surv. Finland 65 (1), 30 pp. [8] vestigations of shallow drill cores have failed to show defi- Pesonen, L.J., 1996. The impact cratering record of Fennoscandia. nite shock metamorphic features (Lehtinen, M., 1993, un- Earth, Moon, and Planets 72: 377-393. publ. data). Petrographic observations of a breccia sample of the new drill core from 156.38 m depth reveal, for the first time, evidence for shock metamorphism. This sample is a very fragile breccia with conical clasts of quartz and granite. Lunar and Planetary Science XXIX 1262.pdf
Saarijärvi Impact Structure, Finland: L. J. Pesonen et al.
Fig.1 (a) Inset: The location and general geology of the Lake Saarij@rvi impact structure in northern Finland (also shown are the seven other known impact structures of Finland (cf. [8])). For rock types, see legend. Note that there is a central island in the lake where two breccia outcrops have recently been discovered [4]. The cross denotes the new drill site. (b) Ground electromagnetic (Slingram, in-phase component) anomalies associated with the Saarij@rvi structure [6]. (c) Multiple sets of PDFs in the breccia layer of the drill core at 156.38 m depth.